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<x-charset iso-8859-1>Hello- > I got a general knowledge sorta question here, regarding engine RPM: > A tach gives a reading based on pulses thru an SP wire... "But", I say to > myself, "the plug only fires every OTHER revolution." > So is my tach telling me distributor RPM, and the crank is actually spinning > twice as fast? The distributor's points fire with every spark plug firing. Directly, the tachometer reads the frequency of the squarewave the points make. So, for a 4-cyl. engine, that's twice per revolution. For a 6-cyl, it's three times, 8-cyl it's four times, etc. Every tachometer must be calibrated for the number of cylinders the engine has, essentially to "divide" this number by 2, 3, 4, etc. to get the true crank rpm. Take care, Shad Laws LN Engineering - Aircooled Precision Performance http://www.LNengineering.com ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/ </x-charset>